The carry-over of Campylobacter strains from one flock to a subsequent flock in the same broiler house has been studied using molecular epidemiological techniques. In all, 524 Campylobacter strains, isolated from two sequential broiler flocks from 60 broiler houses, were typed by restriction fragment polymorphism of the polymerase chain reaction (PCR) product of the flaA and flaB genes (fla typing). Selected strains were also typed using pulsed field gel electrophoresis (PFGE). By fla typing, 15 (21%) of the 60 houses with Campylobacter-positive sequential flocks had identical genotypes. In 10 (16% overall) of these houses the strains were also identical by PFGE. The difference in PFGE patterns in the strains from the three remaining houses may be indicative of genetic instability. Overall, these results suggest that carry-over from one flock to a subsequent flock in the same house is a relatively infrequent event and, therefore, that routine broiler house cleansing and/or disinfection is largely adequate to eliminate Campylobacter contamination. An alternative explanation of the low level carry-over is a persistent source or reservoir, external to the environment of the broiler houses.